1. Field
The present work relates generally to digital communication systems, and more specifically to the multiplexing design for shared control channels in wireless devices.
2. Background
The operation of mobile wireless communication systems requires significant control overhead in both uplink (UL) and downlink (DL) directions. Control signaling effectiveness over a wide range of operating scenarios is oftentimes limited by the availability of over-the-air (OTA) resources and scheduling inflexibility, especially when blocked or preempted by higher priority control signaling or conflicting operations. One possible solution for handling control channel scheduling conflicts involves suppressing the control signaling in conflict. This approach results in associated control loops being temporarily out-of-sync which is undesirable given that control signaling rates are often already critically low for OTA overhead reduction. A second possible solution involves freezing control signaling during conflict. This approach results in delayed signaling for all control loops with potentially even more significant impact to system performance than that of suppressing the control signaling. A third possible solution involves deferring control signaling in conflict. This approach results in as much system performance impact as freezing when the control channel is in full utilization.
Control channels in both downlink (DL) and uplink (UL) consume a non-negligible percentage of available OTA resources that could otherwise be used for data traffic. Scarce OTA resources allocated for use as control channels are typically shared among as many mobile users or functionalities as possible for efficiency at various levels. Parts of the system design comprise different forms and combinations of well-known and widely adopted multiplexing schemes including frequency division multiplexing (FDM), time division multiplexing (TDM), code division multiplexing (CDM), etc.
Design of control channels multiplexing varies depending on characteristics and requirements of signaling. One extreme class of multiplexing design tries to dedicate a part of channel resources for specific control signaling that is essential to system operation (at the cost of potential under-utilization). Examples include dedicated control channels in cdma2000, UMTS/WCDMA, etc.
A compromised class of multiplexing design attempts to improve utilization by guaranteeing channel resources availability among a pre-determined group of users for specific need of control signaling. Examples include shared control channels in cdma2000, UMTS/WCDMA, UMB, etc.
Another extreme class of multiplexing design allows control signaling to block or to preempt existing assignments of channel resources in an on-demand fashion. As it turns out, this class of multiplexing design has become more widely used in many mobile wireless communication systems rather than just in special cases. Many examples of this type of multiplexing design can be found including (1) data traffic suppressed by control signaling, such as dim-and-burst (D&B) and blank-and-burst (B&B) for circuit voice of AMPS and CDMA, and (2) data traffic frozen by control signaling, such as FL control signaling in 802.20 design, and (3) data traffic deferred by control signaling, such as FL control signaling in UMB design, and (4) control signaling suppressed by higher priority signaling, such as the relatively complicated hop permutations in UMB design such as, for example, (i) F-DPICH, F-CQIPICH & F-BPICH puncturing F-DCH, (ii) F-CQIPICH & F-BPICH puncturing FLCS, (iii) R-ACKCH & R-ODCCH puncturing R-ODCH, and (iv) R-ACKCH puncturing R-CDCCH, and (5) control signaling suppressed due to operation design, such as single power amplifier (PA) flavor of dual-antenna Closed Loop-Transmit Diversity (CL-TD) design for the radio link (RL) of UMTS/HSPA, which requires the mobile unit to transmit from only one of two antennas periodically for channel estimate update at the base station. There is no need for phase feedback signaling when the mobile unit is transmitting from only one antenna.
The single primary antenna (PA) flavor of dual-antenna radio link (RL) closed-loop transmit diversity (CL-TD) design for UMTS/HSPA is a simple but effective example for clearly illustrating multiplexing issues of shared control signaling. A simplified diagram of RL CL-TD architecture 100 is shown in FIG. 1. RL CL-TD architecture 100 comprises base station 110, channel 135 and mobile station 140. Base station 110 comprises a receiver (Rx) 115, phase measurement unit 120, phase quantization unit 125 and transmitter (Tx) 130. Mobile station 140 comprises a receiver (Rx) 145, phase de-quantization unit 150, phase control unit 155 and transmitter (Tx) 160. Base station 110 processes a wireless signal received through the channel 135 from the mobile station 140 to estimate phase adjustment for better reception, and quantizes such estimates for signaling back to the mobile station 140, which makes adjustments accordingly.
The RL CL-TD operation requires more than one transmit antenna, but not necessarily more than one primary antenna (PA), when operating over a slowly varying or quasi-stationary channel. FIG. 2 shows one possible way of achieving RL CL-TD with single-PA, by transmission from both primary antenna 210 and secondary antenna 220 for M slots (M=8 in the FIG. 2 example), driven by the same (and the only) primary antenna (PA) 210, followed by transmission only from the primary antenna 210 for N slots (N=3 in the FIG. 2 example), followed by transmission from both antennas (210, 220) again, etc. Periodically turning off the secondary transmit antenna 220 allows the base station 110 to separate responses from two transmit antennas (210, 220) for estimation of phase adjustment.
Assume now a separate code channel of spreading factor (SF) 256 is used for phase feedback signaling among up to ten (10) mobile units in each of 1500 Hz slots, similar to F-DPCH arrangement for RL power control signaling when operating in high speed (HS) mode. The phase feedback signaling rate required depends on a capacity-performance tradeoff of the RL channel scenarios that the CL-TD is intended to handle. In FIG. 3, the letter K represents the number of mobile users that share the phase feedback control channel. FIG. 3 shows examples of phase feedback signaling at 1500 Hz (K=1), 750 Hz (K=2), 500 Hz (K=3), 375 Hz (K=4), 300 Hz (K=5) and 150 Hz (K=10), with suppression during single-antenna transmission periods. For each of the illustrated examples, with the various values of K, FIG. 3 shows the case for only one user currently active.
The lower the phase feedback signaling rate, the greater number of mobile users a single control channel can obviously accommodate. However, the lower the phase feedback signaling rate, the more impact from a single-antenna transmission during which the phase feedback signaling is not available. As can be observed in FIG. 3, with M=8 and N=3, but with only one active user, the interval between two phase feedback signaling time slots is extended by a full period (i.e., a full time slot) each time the phase feedback signaling is blocked. The effect of blocking phase feedback signaling becomes more severe on any given user as the number of users K increases. The effect of blocking phase feedback signaling is most severe when blocked consecutively as in the case of 150 Hz when the value of K is ten (K=10).
The implication is that system performance is significantly degraded when phase feedback signaling is blocked, unless wireless control channel resources are sufficiently over-allocated for phase feedback signaling. Neither situation is desirable, because both imply a sub-optimal tradeoff of system performance.
As mentioned above, one conventional alternative is to temporarily freeze the scheduling of control signaling as if the durations of single-antenna transmission are simply inserted into the timeline as shown in FIG. 4, which depicts the situation of only one active user (to facilitate comparison with FIG. 3). This technique works best if phase feedback signaling is blocked only very briefly, but obviously still leaves room for improvement in the sense that it delays all signaling by the same amount regardless of signaling rate and control channel utilization or system loading.
Another conventional alternative mentioned above involves deferring the scheduling of only the control signaling that is blocked during single-antenna transmission as shown in FIG. 5. FIG. 5 also depicts the situation of only one active user (to facilitate comparison with FIGS. 3 and 4). This technique is able to take advantage of control channel utilization and to delay individual phase feedback signaling by a minimally necessary amount, while still degenerating into the result of FIG. 4 when the system approaches full loading.
Further complications to multiplexing of shared control signaling for single-PA RL CL-TD, which are applicable to shared control signaling in general as well, include the fact that it is potentially desirable to adapt the timing and duration for transmission from the single antenna, adapt the rate of phase feedback signaling, as well as adapting other operation parameters for all mobile versus for individual mobiles, etc.
There is therefore a need in the art for improvements in the multiplexing design for shared control channels in wireless devices.